As a basic study on fatigue damage accumulation of structural members subjected to service load, fatigue tests under superposed stress and varying stress amplitudes have been carried out, and various fatigue life prediction methods have been evaluated.
To investigate the applicability of four different stress cycle counting methods to more complicated stress patterns than a simple superposed stress pattern, the in-plane bending tests were carried out on the specimens made of SCM3 under the combined superposed stresses having such a pattern that the maximum stress amplitudes of two superposed stresses varied stepwise.
The stress cycle counting methods investigated were the zero cross range pair mean method (ZRPM), the range pair mean method (RNPM), the peak method (PEAK), the range pair method (RNPR). The modified Goodman's method was used to evaluate the effect of mean stress on stress amplitude. The lives estimated by using the linear cumulative fatigue damage law were compared with the experimental lives. and the applicability of the counting methods was examined.
The results obtained were follows.
(1) For the superposed stress wave and the combined superposed stress waves, the ratio of the experimental life (N) to the estimated life (Nes) was within the range of 1/4≤N/Nes≤3, when the ZRPM and RNPM methods were applied.
(2) The scatter band of the estimated-experimental life ratio by the ZRPM and RNPM methods was smaller than that of the PEAK or RNPR method.
(3) When the following equation was used to evaluate the effect of mean stress (σm) on stress amplitude (σa),
the ratio of the experimental life to the estimated life was within the range of 1/3≤N/Nes≤3 by the ZRPM and RNPM methods.
σt=σa/(1-σm/kσB), k=σmax/σB+0.2
σt: stress amplitude at zero mean stress (kg/mm2)
σB: tensile strength (kg/mm2)
σmax: maximum stress amplitude of the superposed stress wave (kg/mm2)
k: modified coefficient of σB
